You'd need to define cheap and light. Guess you could plot a chat of weight against cost and there would probably be a sweet spot for you.

Edit: FWIW, I built one up on a scandal frame (Reba, Stans Crest wheels, XT/X9 drivetrain, and some light carbon bits from xcracer) to have a go at the 29er thing, a shade under 23lbs. I reckon you could do something similar for about £1500.

Carbon ideally though not essential
26lbs ish
Forks on the front
29er
Comfortable
£1000 or less

Need a full build? Have you got much of a parts bin?

For a full build maybe a Boardman Pro 29er for a penny short of a grand. Dump the front mech and go 1x to drop a bit of weight of the claimed 12.08kg. Change the nobby nic's for something more racey and tubeless. Should get it down to 25lbs without much effort or cost (especially with some parts swapping off the enigma.)

But it wouldn't have the roll ability of a 29er though, or is that really just marginal?

Edit: bit more of a serious answer, having tried a 29er I'll not go back to racing on a 26.

But it wouldn't have the roll ability of a 29er though, or is that really just marginal?

I think you'd be better off spending money on you than the bike. Pay for a trainer/training program, get some skills training. You'll gain more time int he technical stuff or climbing (especially climbing as you proportionally do more of it) by being good at those than having tyres that roll slightly better on bumps.

markrtw » One of the Cube carbon Reaction series? Don't they start at a grand or so?

I had to take a 27.5 Reaction GTC for a wee test ride and I was stunned at how fast it felt. There's a steep brae up the back of the shop that is good for testing gear changes. The GTC was stomping up a couple of gears faster than normal.

No Scandals and the Dirty Harry would be pushing the budget, would need to be Chinese carbon. Reckon you'd be hard pushed to get a self build in under a grand unless you have a well stocked parts bin. Could you raid the Enigma for parts?

I sold a Scandal/Reba/Hope/Stans rolling chassis for less than £450 last year - was in mint condition. If found somethign similar and you got an XT groupset from Merlin etc you'd be nearly there for less than a grand.

It'll still be heavier than your Ti bike, though, and therefore maybe slower...